DIN stands for Deutsche Industrie Norm. It is the abbreviation for German industrial standards. International industry standards are ISO. In most cases, if something has a DIN standard, it is the same as the ISO standard. Ski binding release is just one thing that is regulated by DIN/ISO standards. Your BMW 3-series conforms to all sorts of DIN standards. So does the lauter tun at the brewery down the road that bought its brewhouse from Bavaria.
The short version is, since binding release is a DIN standard, the amount of torque to release a Tyrolia binding set at 7 is the same to release a Marker, or a Salomon, or a Look. That's the expectation, at least.
Now, that means the release will work the same the day that binding comes off the production line. As the binding ages, it won't work exactly as it did when it was brand new. The materials in the binding will slowly degrade, and the torque to release the binding at a given setting will gradually become less and less. An 8 year old binding set at 7 might release at the torque level of a new binding set at 4, for example. Eventually, the materials break down to the point the binding is no longer serviceable. That's when the binding drops off the manufacturer's indemnity list, and no shop will touch it anymore.
As far as how it is calcluated, it's all calculated with kilonewtons of force. Each number on the DIN scale equates to a certain number of kilonewtons to make that binding release. If you want to figure out what those values are, you can probably take your skis into a shop and have them torque tested. If you ask for the test results, a modern Wintersteiger tester gives the kilonewton results for each release. Check your DIN setting, and you know x DIN= y kN. If you can get the kN value for a second DIN, you could probably extrapolate that out across the entire DIN scale.